The present invention relates to the field of data transmission, such as data transmission that may occur in an optical network. More particularly, it pertains to a method and apparatus for providing a more efficient use of the total bandwidth capacity in a synchronous optical network.
Within the ever-evolving telecommunications industry, the advent of numerous independent, localized networks has created a need for reliable inter-network communication. Unfortunately, this inter-network communication is difficult to accomplish in a cost-effective manner due to differences in the digital signal hierarchies, the encoding techniques and the multiplexing strategies. Transporting a signal to a different network requires a complicated multiplexing/demultiplexing, coding/decoding process to convert the signal from one scheme to another scheme. A solution to this problem is SONET, an acronym for Synchronous Optical NETwork. It is an optical transmission interface, specifically a set of standards defining the rates and formats for optical networks. Proposed by Bellcore during the early 80s and standardized by ANSI, SONET is compatible with Synchronous Digital Hierarchy (SDH), a similar standard established in Europe by ITU-T. SONET offers a new system hierarchy for multiplexing over modern high-capacity fiber optic networks and a new approach to Time Division Multiplexing (TDM) for small traffic payloads. SONET has several advantages, including:
meeting the demands for increased network Operation and Maintenance (OAM) for vendors and users by integrating the OAM into the network, thus reducing the cost of transmission;
standardizing the interconnection between different service providers (Mid-Span Meet);
allowing the adding and/or dropping of signals with a single multiplexing process, as a result of SONET""s synchronous characteristic.
The Synchronous Transport Signal (STS) frame is the basic building block of SONET optical interfaces, where STS-1 (level 1) is the basic signal rate of SONET. Multiple STS-1 frames may be concatenated to form STS-N frames, where the individual STS-1 signals are byte interleaved. The STS frame comprises two parts, the STS payload and the STS overhead. The STS payload carries the information portion of the signal, while the STS overhead carries the signaling and protocol information. This allows communication between intelligent nodes within the network, permitting administration, surveillance, provisioning and control of the network from a central location. At the ends of a communication system, signals with various rates and different formats must be dealt with. A SONET end-to-end connection includes terminating equipment at both ends, responsible for converting a signal from the user format to the STS format prior to transmission through the various SONET networks, and for converting the signal from STS format back to the user format once transmission is complete.
The optical form of an STS signal is called an Optical Carrier (OC). The STS-1 signal and the OC-1 signal have the same rate. The SONET line rate is a synchronous hierarchy that is flexible enough to support many different capacity signals. The STS-1/OC-1 line rate was chosen to be 51.84 Mbps to accommodate 28 DS1 signals and 1 DS3 signal. The higher level signals are obtained by synchronous multiplexing of the lower level signals. This higher level signal can be represented by STS-N or OC-N, where N is an integer. Currently the values of N are 1, 3, 12, 48 and 192. For example, OC-48 has a rate of 2488.320 Mbps, 48 times the rate of OC-1.
Existing optical networks can be formed by several inter-connected rings, each ring formed itself by several nodes connected to one another. In a Bi-directional Line Switched Ring (BLSR), there exists between every two nodes of the ring both working and protection bandwidth. In the situation where the working bandwidth fails, the protection bandwidth is used to perform data transmission. In the situation where both working and protection bandwidth fail, the data transmission is re-routed around the ring using the protection bandwidth available between the other pairs of nodes within the ring.
In a four-fiber BLSR, two lines connect neighboring nodes, a working line and a protection line. The working line provides the working bandwidth and the protection line provides the protection bandwidth. Each line is formed of two fibers, one for each direction of traffic flow. Thus, the working line includes a send working fiber and a receive working fiber, while the protection line includes a send protection fiber and a receive protection fiber. The term xe2x80x9cbi-directionalxe2x80x9d of BLSR refers to the fact that if one fiber of the working line fails, or if a piece of equipment to which one fiber of the working line is connected fails, traffic for both directions is re-routed. Specifically, if a working line suffers a data transmission impairment, either a fiber failure or an equipment failure, a span switch allows the protection line to be used as an alternate route of transmission. If both the working line and the protection line fail (link failure), or should there be a node failure, a ring switch allows for the data transmission to be re-routed around the ring via the other nodes in the ring network, specifically over the different protection lines. Both the span switch and the ring switch are different forms of protection switching.
Optical networks such as the BLSR are no longer used simply to transmit voice data, but rather are now carrying more and more pure data such as Internet traffic in addition to voice data. Network users are demanding greater bandwidth capacity and are requiring less and less protection of the data transmissions, due to the very nature of the Internet, within which routers take care of re-routing traffic when failures occur.
One solution to provide greater bandwidth capacity currently in implementation is the use of stacked overlaid BLSRs. For each node within a BLSR, a second (sister) node is installed at the same site. The two nodes at each site are inter-connected using new fibers and exchange complicated signaling control information. In addition, the new nodes are all inter-connected by a second ring using new fibers, thus forming a second, stacked ring. Unfortunately, this solution is very expensive to implement and is still limited with respect to the amount of working bandwidth available to customers, due to the reservation of one protection fiber for each working fiber.
Another solution is the implementation of a mesh network, in which any one node may be connected to any other node of the network. Although this solution is theoretically proven to be less expensive to implement than a BLSR and to provide greater bandwidth capacity to network users, it becomes very complicated to provide an adequate level of protection within the mesh network.
The background information provided above clearly indicates that there exists a need in the industry to provide a method and apparatus for increasing the degree of utilization of the total available bandwidth in optical networks such as to either transmit more data or reduce the infrastructure necessary to transmit the same amount of data.
The present invention provides in one aspect a local node for use in a synchronous optical network ring. The local node includes a group of working transmission lines for exchanging data with a remote node in the network, and a single protection line associated with the group of working transmission lines for exchanging data with the remote node in the event of a data transmission impairment on any one of the working transmission lines. The node is operative to monitor the working transmission lines and, upon detection of a transmission impairment over any one of the working transmission lines, invoke a protection switch event whereby the traffic normally sent over the working transmission line that suffers the impairment is re-routed over the protection line. This protection switch event is referred to as a span switch.
The local node as described above yields either one of two possible benefits. If the user requires an increase of bandwidth capacity, this can be accomplished by converting an existing protection line to a working transmission line. On the other hand, if a reduction in the infrastructure is desired, while maintaining the existing working transmission line capacity, this can be accomplished by reducing the number of protection lines with respect to the number of working transmission lines.
It should be appreciated that the invention is not limited to a single protection line per local node. The local node may comprise a plurality of protection lines where each protection lines services a group of working transmission lines.
In this specification, xe2x80x9cdata transmission impairmentxe2x80x99 refers to a condition that either negates or reduces the ability of a working transmission line to carry data to the intended destination. A xe2x80x9cdata transmission impairmentxe2x80x9d occurs when a fiber is cut, or intermediate equipment malfunctions such as to totally interrupt the data traffic, also referred to as a fiber failure. A xe2x80x9cdata transmission impairmentxe2x80x9d also occurs when the fiber or intermediate equipment is rendered partially inoperative such that not all traffic is lost, but the normal capabilities of the working transmission line are significantly diminished. Further, a xe2x80x9cdata transmission impairmentxe2x80x9d occurs when a node within the network becomes inoperative, also referred to as a node failure, or when the link connecting two adjacent nodes within the network becomes inoperative such that no traffic may be exchanged between the two nodes over any one of the working transmission lines and protection line, also referred to as a link failure.
In a specific example of implementation, each working transmission line includes a send connection for sending optical signals to the remote node and a receive connection for receiving optical signals from the remote node. A data transmission impairment detected over a particular working transmission line may be a malfunction over either one of the receive and send connections of the particular working transmission line.
Since a single protection line is available to protect multiple working transmission lines, it has been found advantageous, although not necessarily essential, to the invention to provide each group of working transmission lines that connects the node to an adjacent node in the network ring with a user-defined priority scheme. In a specific non-limiting example of implementation, the priority scheme assigns a priority level to each working transmission line of the group. In the case of fiber failures over multiple working transmission lines between two adjacent nodes, protection switching is implemented on the basis of the priority scheme.
In a specific non-limiting example of implementation of the invention, the protection line also serves to implement a different type of protection, notably ring protection. Ring protection ensures that if a link failure occurs between the node and a first adjacent node (i.e. all working lines and protection line suffer from a data transmission impairment) or a node failure occurs at the first adjacent node, an alternate route will be used in order to ensure traffic flow. This alternate route is via a second adjacent node and, subsequently, the other nodes within the network ring, using the available protection bandwidth.
In a specific non-limiting form of realization, a local node implementing the principle of the invention is one component of a synchronous optical network, where this network comprises a ring inter-connecting two remote telephone instruments (also referred to as Customer Premises Equipment (CPE)). The telephone instruments are therefore the end-points for a SONET connection. Alternatively, the end-points for the SONET connection could be the modems of two remote computers. The ring is an OC-192 ring, where the optical signal being transmitted within each ring is an OC-192. Alternatively, the end-points may be inter-connected by multiple rings of various types, for example an OC-48 ring and an OC-192 ring. The local node is connected to a remote, adjacent node by three lines, two working transmission lines and a protection line. Each line is implemented by a fiber pair, one fiber for each direction of traffic flow, thus implementing both a send and a receive connection. The working transmission lines are regularly used for the exchange of traffic between the two adjacent nodes. If the send or receive fiber of a working transmission line should suffer a data transmission impairment, the protection line will assume transmission duties for this working transmission line.
In a specific non-limiting example of implementation, the local node is analogous to a computing device structurally comprised of a control unit and several interfaces, the control unit itself including a memory and a processor. An internal system bus interconnects these components, enabling data and control signals to be exchanged between them. The interfaces interconnect various bi-directional ports to their respective physical paths, including both the working transmission lines and the protection line, such that the local node may exchange data with remote, adjacent nodes.
The memory contains a program element that controls the operation of the local node. This program element is comprised of individual instructions that are executed by the processor, implemented in the form of a Central Processing Unit (CPU). In addition, the memory provides random access storage, capable of holding data elements that the controller manipulates during the execution of the program. For all transmission nodes within SONET rings, the execution of the program element by the processor ensures standard data transmission and error/failure monitoring, including the multiplexing and de-multiplexing of optical signals as well as standard protection switching support.
Specific to a non-limiting example of realisation of the present invention, the execution of the program element stored in the memory of the local node ensures span and ring switching on the basis of a single protection line available to multiple working transmission lines between the local node and its remote, adjacent nodes. Accordingly, the memory also supports a user-defined priority table that maps a priority level to each working transmission line connected to the local node for exchanging data with adjacent nodes, grouped by transmission span. Note that both fibers of a particular working transmission line are assigned the same priority level.
In one possible form of implementation, the control unit itself is responsible for monitoring the working transmission lines for the presence of a data transmission impairment, where such an impairment could result in the loss of intraring data incoming from an adjacent node. This verification may be effected by constantly monitoring incoming lines for manifestations of data transmission impairments that indicate a loss of data. Examples of manifestations of data transmission impairments are Loss of Signal, Loss of Pointer, Line Alarm and Path Alarm. These data transmission impairments are reported in the SONET overhead. When the control unit detects a data transmission impairment, be it a fiber failure, a node failure or a link failure, the control unit responds to this data transmission impairment by invoking a protection switch event.
In the case of a fiber failure, the protection switch event could be a span switch. In the situation where the control unit detects multiple simultaneous fiber failures over different working transmission lines between the node and a particular adjacent node, the control unit consults the priority table to determine which of the working transmission lines is to be protected. The control unit then invokes the span switch for the working transmission line having the highest priority level. The working transmission line that goes unprotected due to a lower priority level is squelched by the control unit, whereby the control unit generates an error signal, predetermined within the network ring as being representative of a fiber failure. This error signal is sent back to the end points of the SONET connection (CPEs), such that the end points are informed of the data transmission impairment within the network. In a specific non-limiting example, the error signal is a particular sequence of bits.
In the case of a link or node failure, the protection switch event is a ring switch. When the control unit detects a link or node failure, it consults the priority table to determine which working transmission line among those affected by the data transmission impairment is to be protected. Whether the data transmission impairment is a link failure between the node and a particular adjacent node or a node failure at a particular adjacent node, the control unit determines from the priority table the working transmission line having the highest priority level for the group of working transmission lines corresponding to the transmission span between the node and the particular adjacent node. The control unit next invokes the ring switch for the working transmission line having the highest priority level, such that data transmissions over this working transmission line are re-routed around the ring using the available protection bandwidth. As described above, the control unit squelches a working transmission line that goes unprotected due to a lower priority level.
Note that for both a span and a ring switch, all nodes of the network ring are advised of the protection switch, through signaling information generated by the particular node that detects the data transmission impairment, be it a fiber, link or node failure, and implements the protection switch. This signaling information provides details as to the type of data transmission impairment, as well as to the particular working fiber (and thus working transmission line) that is being protected by the protection switch.